Depression chains in seafloor of contrasting morphology, Atacama Trench margin: a comment on Marshet al.(2018)

Abstract

This comment presents acoustic and visual data showing deep seafloor depression chains similar to those reported in Marsh et al. ( R. Soc. open sci. 5: 180286), though from a different deep-sea setting. Marsh et al. present data collected during cruise JC120 from polymetallic nodule rich sites within the Clarion-Clipperton Fracture Zone (CCFZ), at water depths of between 3999 and 4258 m. Within this comment, we present data collected with equivalent acoustic and imaging devices on-board the RV Sonne (SO261—March/April 2018) from the Atacama Trench, approximately 4000 m depth, which shows comparable depression chains in the seafloor. In contrast with the CCFZ observations, our study area was wholly free of polymetallic nodules, an observation therefore weakening the ‘ballast collection’ by deep-sea diving mammals formation hypothesis discussed in their paper . We support their alternate hypothesis that if these features are indeed generated by deep-diving megafauna, then they are more likely the resultant traces of infauna feeding or marks made during opportunistic capture of benthic fish/cephalopods. We observed these potential prey fauna with lander and towed camera systems during the cruise, with example images of these presented here. Both the SO261 and JC120 cruises employed high-resolution sidescan systems at deployment altitudes seldom used routinely until the last few years during scientific deep-sea surveys. Given that both cruises found these depression chains in contrasting physical regions of the East Pacific, they may have a more ubiquitous distribution than at just these sites. Thus, the impacts of cetacean foraging behaviour on deep seafloor communities, and the potential relevance of these prey sources to deep-diving species, should be considered.

The COSMUS Expedition: Seafloor Images and acoustic bathymetric data from the PS124 expedition to the southern Weddell Sea, Antarctica

“…Unfortunately, during PS124 problems with communication with the camera resulted in fragmented video files and occasional gaps in video data collection. Despite the breakup in the video data, the collected fragments may be used for the 3D 'Structure from Motion' reconstruction of areas of high resolution seafloor topography, an approach which has recently been used with OFOBS data to demonstrate the movement of Arctic sponges (Morganti et al, 2021) and the interactions of deep diving whales with the deep sea seafloor (Purser et al, 2019a).…”

“…This data can then be used, in combination with the OFOBS positioning information, to generate 10 and 20 cm resolution topographic maps of the swathes of seafloor passed over by the OFOBS during deployments. The low deployment height of the side scan system allows small surface depressions and features of a few 10s of cm height to be visible in the derived mapping products, to elucidate aspects of seafloor structure and fauna interactions with the seafloor on a high resolution (Purser et al, 2019a;Marsh et al, 2018). Maps at 10 and 20 cm resolution have been produced for all station deployments, and form part of this data set.…”

The COSMUS expedition: seafloor images and acoustic bathymetric data from the PS124 expedition to the southern Weddell Sea, Antarctica

“…Unfortunately, during PS124 problems with communication with the camera resulted in fragmented video files and occasional gaps in video data collection. Despite the breakup in the video data, the collected fragments may be used for the 3D "structure from motion" reconstruction of areas of highresolution seafloor topography, an approach which has recently been used with OFOBS data to demonstrate the movement of Arctic sponges (Morganti et al, 2021) and the interactions of deep-diving whales with the deep seafloor (Purser et al, 2019a).…”

“…These data can then be used, in combination with the OFOBS positioning information, to generate 10 and 20 cm resolution topographic maps of the swathes of the seafloor passed over by the OFOBS during deployments. The low deployment height of the side-scan system allows small surface depressions and features of a height of a few tens of centimetres to be visible in the derived mapping products to elucidate aspects of the seafloor structure and fauna interactions with the seafloor at a high resolution (Purser et al, 2019a;Marsh et al, 2018). Maps at 10 and 20 cm resolution have been produced for all station deployments and form part of this dataset.…”

First in situ observations of the benthic-demersal fauna on the upper continental slope off Punta Pichalo (19°36'S), northern Chile

“…Nearly all investigations of the deep-sea benthos off Chile used traditional gears such as dredges and bottom Agassiz trawls, except the PUCK (SO-156) Expedition, using additional underwater video using a remotely operated vehicle (ROV). Underwater imagery was also used to document the seafloor of the Atacama trench during the 2018 cruises Atacamex and RV Sonne SO261 (Purser et al 2019), and recently in January 2022, a manned submersible explored the abyssal of the Atacama trench. However, the upper continental slope off northern Chile was excluded from these recent studies.…”